The present invention relates to a current sensor and a PWM (Pulse Width Modulation) inverter that uses the current sensor, and particularly relates to a current sensor which is suitable for detecting currents flowing in both the positive and negative directions and a PWM inverter which incorporates said current sensor.
Conventionally in automobiles, a power MOSFET (Metal Oxide Semiconductor Field Effect Transistor), which is a switching element for controlling electrical components of the automobile, is used to control current flowing through electrical components. If a current flows for a long time or an overcurrent occurs, the power MOSFET itself generates heat causing a thermal breakdown to occur. Furthermore, in a PWM inverter that uses a power MOSFET for controlling three-phase alternating current, current flows through the power MOSFET in two directions: from a drain electrode of the power MOSFET to a source electrode, and the reverse direction.
Therefore, for example, as disclosed in the Japanese Application Patent Laid-open Publication No. Hei 06-351280, a technique is known that utilizes an insulated current sensor that uses a Hall element to detect current or utilizes a shunt resistor and a sample-and-hold circuit to detect current.
However, a current detection means that uses an insulated current sensor or uses both a shunt resistor and a sample-and-hold circuit requires a separate device or circuit, resulting in increasing both the current sensor cost and the size of the PWM inverter, which is a problem.
On the other hand, for example, another known technique has been disclosed in the Japanese Application Patent Laid-open Publication No. 2000-193692. Therein, a series circuit of a mirror MOSFET and a current detection resistor is connected in parallel to the power MOSF ET, and a voltage across the current detection resistor is compared with the reference voltage by a comparator, and if a voltage that exceeds the reference voltage has been applied to the current detection resistor, it is considered that an overcurrent has flown through the power MOSFET, and the current is shut down to prevent the power MOSFET from being damaged.
However, there is a problem that the current detection method described in the Japanese Applicatio n Patent Laid-open Publication No. 2000-193692 cannot be applied to the current detection of the PWM inverter because it is impossible for the method to detect both a current in the positive direction and a current in the negative direction. That is, a current sensor that uses a comparator in addition to a mirror MOSFET and a current detection resistor that are connected in parallel to a power MOSFET is a means for detecting an overcurrent when a current flows through the power MOSFET from the drain electrode to the source electrode. Therefore, it is difficult to detect a reverse current, that is, a forward current flowing through a parasitic diode of the power MOSFET. For example, when a current flows in the same direction as the forward current flowing through the parasitic diode of the power MOSFET, the voltage signal of the current detection resistor becomes negative, and when a current flows from the drain electrode of the power MOSFET to the source electrode, the signal becomes positive. It is difficult for the comparator to detect both of those voltage signals, and therefore, there is a problem that the method cannot be applied to the current detection of the PWM inverter.
Furthermore, when a current flows through each parasitic diode of the power MOSFET and the mirror MOSFET, the relationship between the current value and the voltage drop value of each parasitic diode is non-linear, and the current detection resistor is linear. Therefore, when a current flows through each parasitic diode, the ratio of the current flowing through the power MOSFET to the current flowing through the mirror MOSFET varies according to the value of the flowing current, which may cause an area in which the voltage drop value of the current detection resistor becomes almost equal to that of the power MOSFET. In this area, regardless of an increase or decrease of the current that flows through the power MOSFET, the current flowing through the mirror MOSFET does not change. As a result, in this area, it is not possible to accurately detect current that flows through the power MOSFET.